Extending a classes hook with DataObjectDecorator

Hi,
I want to augment the Member classes functionality wit the DataObjectDecorator by adding new methods and properties. I also want to augment the onAfterWrite() hook with the DataObjectDecorator but i presume that the existing hook of the Member class will be overwritten if I define the hook on the DataObjectDecorator.
How can I add something to a hook via a DataObjectDecorator?

You can add methods that don't already exist, but you can't override methods. If a method calls $this->extend(), you can provide a method that'll get called then (the name of the method you can provide is the first argument).